1. Technical Field
The invention relates to an antireflection film that is formed on a bonded surface between optical members such as a lens and a filter and exerts an antireflection effect with respect to light in a predetermined band; an optical element; and an optical system including the same.
2. Description of the Related Art
In general, an imaging apparatus such as a photographing camera and a broadcasting camera has a plurality of optical members, such as lenses, prisms, and filters, disposed on its light path. When the optical members are bonded to each other through an adhesion layer made of a transparent resin, a part of incident light may be reflected at the adhesion interface between the adhesion layer and the optical members, thereby causing flare or ghost appearing on an image. Further, reflectivity at the adhesion interface has distribution depending on the wavelength of the incident light, and also shows different wavelength dependency in accordance with constituent materials of the respective optical members. Therefore, chromaticity balance is degraded, and it is necessary to adjust the white balance of the entire imaging apparatus.
Accordingly, an antireflection film is provided between the optical members and the adhesion layer.
The antireflection film is a multilayer film in which a plurality of dielectric films having different refractive indices from each other are combined. JP 2001-74903 A and JP 2006-284656 A disclose antireflection films.
Recently, however, an optical system in which a glass having a high refractive index Nd of around 2.0 with respect to d line (wavelength λ=587.56 nm) is frequently used. In the antireflection films disclosed in JP 2001-74903 A and JP 2006-284656 A, there are difficulties in sufficiently reducing reflectivity at the upper and lower limits of a band that is thought to be a visible-light region (that is, around 400 nm and around 700 nm). Therefore, such a method has been considered that reduction in reflectivity is achieved by an adhesion layer made of a resin material having a higher refractive index. Actually, however, there exists no proper resin having a refractive index Nd of more than 1.6. Therefore, even when a glass having a refractive index of around 2.0 is bonded, an antireflection film which that exert excellent light transmittance is demanded.